1. Field of the Invention
This invention relates to adaptive control in which the response of a feedback control automatically adjusts to gain variations in the controlled system. More particularly the invention pertains to turning machine tools such as horizontal and vertical lathes. Specifically, the invention relates to machine tools with an adaptive control, in which machining parameters such as workpiece surface velocity at the cutting edge of the tool and tool feed velocity are continuously adjusted during machining in response to measured machining parameters during the machining operation.
2. Description of the Prior Art (Prior Art Statement Under Rule 97)
It is well known in the art that machining time may be decreased by controlling machine tool drive and feed inputs to maintain certain machining parameters at maximal limits. One such limit is the workpiece surface velocity at the cutting edge of the cutter tool, measured in surface feet per minute and thus known in the art as SFM. Excessive SFM may burn the tool, and thus tool makers specify a maximum SFM for a given tool. Mack, U.S. Pat. No. 3,840,791 issued Oct. 8, 1974, for example, describes a system in a turning machine for accelerating a rotating workpiece as the cutter at first approaches the workpiece in order to obtain a maximal SFM at the beginning as well as throughout the machining operation. Another such limit is the power to the machine drive, measured in horsepower and known as HP. It is known that the maintenance of a maximal HP is desirable to minimize machining time, absent other limiting factors.
It is also known that there are certain ranges of machining parameters that constrain the selection of maximal SFM or HP. One such parameter is the tool feed velocity into the workpiece, measured in inches per revolution for turning machines and thus known as IPR. IPR is a measure of the depth of penetration of the tool into the workpiece and thus the width of the shavings or "chip thickness" of material cut from the workpiece. The maximum permissible force on the tool tip sets the maximum IPR, and exceeding this maximum might cause chatter, or tool breakage. On the other hand, if the IPR is below a certain minimum, depending on the tool geometry (especially its "chip breaker") and workpiece material, a large mass of thin, curled strips will be produced at the cutter and may clog the machine. Normally, the turning machine is operated at or above this minimum IPR so that the cutter breaks the shavings into small chips that fall away clearly and are easily removed from the machine.
There are many well known mathematical techniques, such as linear programming, for optimizing a function of several parameters, functionally related in a known way, within given constraint boundaries. For machining parameters, however, it is known that many of the functional relations between controllable and measured parameters change throughout the machining operation due to dulling of the cutter edge and changing geometry and surface condition of the workpiece. Thus, it is known in the prior art that the optimization procedure should adapt to the changing conditions of the machining parameters and the functional relations between them.
The prior art methods of adaptive control, such as that disclosed by B. Beadle et al. in U.S. Pat. No. 3,784,798, issued Jan. 8, 1974, assumes that the functional relations between measured parameters, controllable parameters, and the parameters sought to be optimized maintain certain general characteristics despite the fact that the functional relationships change. In particular, an increase in a measured parameter results from either an increase or decrease of controllable parameters, and an increase in the parameter to be optimized results from either an increase or decrease of the controllable parameters. Thus, optimization may proceed in a stepwise fashion of incrementally changing controllable parameters most likely to increase the parameter to be optimized, but changing controllable parameters in a different fashion most likely to avoid a constraint in measured parameters when a constraint boundry is passed. This method homes in on and iteratively checks for a desired operating point and may oscillate back and forth across constraint boundaries.
Grinding machines have been provided with controllable feeds activated in response to changes in current, power, or speed of the motor providing the transverse cutting velocity. W. Muller, Great Britain Pat. No. 782,432 issued Sept. 4, 1957 discloses an automatic grinding machine having a feed that is turned on and off in response to whether the grinding motor current or speed is less or greater, respectively, than a preset current or speed threshold. Muller also employs a rapid idle traverse to initially advance the grinding disc up to the workpiece, with contact detected by the resultant increase in motor current or drop in motor speed. R. Lenning, U.S. Pat. No. 3,589,007 issued June 29, 1971, discloses a grinding wheel with its feedrate controlled in response to the force exerted on the grinding wheel when it is in cutting engagement with the workpiece. The force is proportional to the power supplied to the grinding motor, and the difference between the measured force and a preselected reference force is used to energize the feed. When the workpiece is reduced to a predetermined size, the reference force is changed. It should be noted that none of these systems responsive to feed motor power determine the actual power delivered to the cutting interface, for example, by correcting for acceleration. Thus the response of these control systems must be limited to insure stability.
Cutting efficiency of rotary saws for slicing semiconductor wafers has been monitored to measure the condition of the edge of the saw. R. Demers, et al., U.S. Pat. No. 4,228,782 issued Oct. 31, 1980, discloses a system for controlling the blade-to-boule feedrate of a wafer cutting saw so that the blade-to-boule force, measured by a mechanical force transducer, is maintained generally constant. The measured blade-to-boule force is compared to a reference force and the error is applied to a feedrate control. During the cutting of the initial and final edges of the wafer the force selection is overridden by a maximum rate-of-feed limitation which is intended to prevent excessive shock and too rapid penetration during initial contact, which could otherwise damage the saw or chip the boule surface due to excessive pressure. The cutting efficiency of the saw is measured by the rate of feed, and the time required to cut a wafer is used as a criterion for determining when the saw blade needs to be dressed or replaced.
The measurement of the electrical power consumed by the drive motor of a turning machine has been used to sense tool breakage and to regulate tool feedrates. Commercial systems are available, for example, that decrease the feedrate if the drive motor power is above a preset maximum threshold, and increase the feedrate if the drive motor power is below a preset minimum threshold. The maximum threshold is set above the minimum threshold, defining a window over which the gain of the feedback control loop is set to zero in order to insure stability. The rate at which the feedrate changes, or response factor, is a predetermined constant, preferably set to the largest possible value which will allow stable operation of the machine. In such machines provisions are made for the user to select a number of high or low measured power limits and associated limit delays in order to control user defined machine functions enabled if the high or low limit is reached continuously for a time period longer than the associated limit delay. A low limit, for example, could be used to detect tool breakage. Moreover, a predetermined machine high power limit is usually provided as a protection feature to shut off the machine if the machine high power limit is exceeded for a time greater than a predetermined machine limit time delay. A somewhat shorter predetermined spindle surge time may be specified to disable the user selected limits when the machine high limit is exceeded, since temporary power surges are randomly encountered during normal machining and thus should be discounted when drive power is monitored to control user functions. An idle power predetermined maximum limit is typically provided along with a predetermined initial "air cut" feedrate and a preset transitional "impact" feedrate so that the tool is first brought into contact with the workpiece at a rapid "air cut" feedrate, and contact is detected by the increase in measured drive power above the idle power limit, whereupon the feedrate is switched to the impact feedrate and held for a predetermined impact hold time before the feedrate control loop is established. Moreover, a predetermined minimum feed limit is typically provided in terms of a percentage of a predetermined initial feedrate to set a floor below which the feedrate will not fall during normal operation of the machine. The combination of a high minimum feed limit and a low machine high power limit possibly could shut off the machine if a tool becomes dull, but at the expense of limiting the permitted operating ranges of workpiece and tool parameters. The use of a low machine high power limit, for example, is particularly ineffective in detecting tool dulling if feedback control reduces the machining rate to maintain a generally constant power in response to tool dulling.